In recent years, a great deal of attention has been given to improving techniques for cleaning surfaces. Surfaces are often cleaned to remove contamination in the form of film-like processing residues (cutting or lubricating oils, mold release materials, salts and oils from finger prints received in handling, etc.). Film-type residues are typically removed by solvent processes, using fluid baths, sprays, vapor cleaners or other methods and apparatus well known and used commercially. Such solvent processes also remove other types of contamination in the form of particulate materials that may include metal, ceramic or polymeric fragments created during a manufacturing process, or deposited by environmental (e.g., airborne) contamination.
In certain applications, the need for improved levels of cleanliness have become more stringent, and traditional methods of cleaning, such as the use of solvent baths, have proven unable to provide satisfactory results. This fact is particularly true in the electronics industry where effective removal of submicron particulate contamination greatly affects the yield of high resolution electronic devices such as integrated circuits as well as a number of other products of commercial importance.
As the size of a particle to be removed from a surface decreases, the removal of such particles becomes increasingly difficult. For this reason, conventional approaches that make use of solvent dips or washes, or fluid streams, lose effectiveness as particle size falls substantially below 1 micrometer (micron or .mu.m). The relative force of adhesion of these particles rises exponentially as the particle size decreases. Table I, below, reports relative adhesion force as a function of particle size.
TABLE I ______________________________________ Particle Adhesion of Glass Beads on a Glass Slide Particle Size Relative Force to displace (.mu.m) (gravitational units) ______________________________________ 100 510 50 2,159 10 57,716 1 674,600 0.1 749,552,300 ______________________________________
Source: "Particulate Removal with Dense CO.sub.2 Fluids," D. Zhang, D. B. Kittelson, B. Y. H. Liu, 1992, presented by McHardy at the Third International Workshop on Solvent Substitution, Phoenix, Ariz., 1992.
As the force necessary to break the combined adhesion and binding charges of a particle rises, the amount of force that can be transferred to the particle by a fluid stream remains constant due to the fixed cross-sectional area of the particle. Additionally, boundary layer effects near tile surface, taking the form of laminar flows of the cleaning gases or fluids over the particles, further isolate microscopic particles from removal. Thus, as the particle size decreases, the ability to displace such particles with fluid streams falls off to the point that it becomes nearly impossible to remove microscopic particulates by spraying the particles with streams of solvent.
In recognition of the ineffective physics of high pressure gas or solvent streams for particulate removal, other investigators have used the kinetic energy of droplet sprays or finely divided solids to remove particulates by means of momentum transfer. Such droplets typically comprise water or CO.sub.2 snow sprays that are directed with a specialized nozzle onto the workpiece being cleaned. Often high pressure driving gases are used to accelerate the snow spray to a sufficient velocity to clean the workpiece.
These cleaning methods are similar to methods wherein naturally occurring sand or manufactured abrasive grit is used as a blasting agent for descaling and cleaning purposes. The principles involved in heavy cleaning or stripping applications consist of supplying sufficient kinetic energy of impact to the blasting agent in order to exceed the adhesive or cohesive strength of the material being removed or abraded and are well known to those skilled in the art. The use of sublimable or phase-changing materials (e.g., water or dry ice) instead of sand or grit allow for easier cleanup and eliminate residues of hard particles, which could damage products in later use.
These aggressive blasting applications, which can produce heavy material abrasion or wear, lie at one end of a continuum of related cleaning processes. At the other end of the continuum are fine sprays of liquid and/or finely divided solid matter, which carry modest levels of kinetic energy. Fine sprays of comparatively low energy, particularly with liquids or low hardness materials such as ice or CO.sub.2 snow, cause little or no damage to surfaces, and are known to be effective for gentle cleaning.
However, as shown earlier in Table I, the binding forces for very finely divided matter disposed on a surface increase exponentially as the size of the particles decreases. With some materials, the binding energies of fine matter on surfaces begin to approach the cohesive strength of the underlying matter under treatment. As impinging sprays are made more aggressive by enlargement of the droplet mass or velocity (or both) in attempts to improve submicron particulate removal efficiency, the threshold level for damage to the underlying material is approached.
One difficulty of cleaning methods that do not control blast particle size closely (e.g., single-stage phase transformation at an orifice) is that a range of droplet sizes and mixed phases are present. A substantial amount of the sprayed material is too fine and fugitive to have any effect, yielding low efficiency, and calling for higher and higher pressures for accelerating gases. At the same time, the presence of solid or liquid phase material in larger masses may produce damage to the surfaces being cleaned with high driving pressures.
The use of high driving gas pressures also accelerates the sublimation of solid phase CO.sub.2, providing low efficiency in the use of the material as a cleaning agent. In addition, the use of a driving gas such as high pressure air or nitrogen adds complexity, and creates additional opportunities for the introduction of impurities.
To summarize the existing art, the velocity of spray or blasting particles has been varied using the following known methods:
1. Use of air or another driving gas to accelerate pellets or sprayed matter, in a manner analogous to sand blasting. PA0 2. Use of the dynamics of liquid spray from siphon-type bottles, wherein the driving force may be the vapor pressure of the liquefied or pressurized gas in the storage cylinder. Siphon pressure may be augmented by an additional head pressure (e.g., as in supercritical fluid extraction grade CO.sub.2, which is supplied with a 2,000 psi head of helium). PA0 3. Airless sprayers to deliver high speed droplets. PA0 4. Centrifugal force to accelerate pellets of CO.sub.2 ice so that pellets can be ejected from a centrifuge at high velocity.
All of these methods suffer from one of two problems. Either the particles sprayed on the workpiece are so hard that the workpiece may be damaged or, if the sprays are softer, the equipment required to accelerate the spray are dangerous and a possible source of contamination. Thus, there is a need for a new method of controlling the kinetic energy of cleaning sprays impinging on surfaces that alleviates some of the shortcomings in the art, as now known and practiced.